Method and apparatus for cooling a rotor assembly

ABSTRACT

A rotor assembly for an electric machine is disclosed. The rotor core has a cylindrical body defining an outwardly facing peripheral surface comprising a slotted portion including a set of slots defined by a set of rotor teeth projecting outwardly from the peripheral surface. Each rotor tooth comprises a respective first distal tip and a respective first radial length extending radially from a center point of the rotor core to the first distal tip. The peripheral surface further comprises a non-slotted portion defining a respective second radial length, extending radially from the center point of the rotor core to the outwardly facing peripheral surface. The first radial length is less than the second radial length.

BACKGROUND

Electric machines, such as electric motors or electric generators, areused in energy conversion. Such electrical machines operate through theinteraction of magnetic fields, and current carrying conductors generatethe force or electricity respectively. Typically, an electrical motorconverts electrical energy into mechanical energy. Conversely, anelectrical generator converts mechanical energy into electrical energy.For example, in the aircraft industry, it is common to combine a motormode and a generator mode in the same electric machine, where theelectric machine in motor mode functions to start the engine, and,depending on the mode, also functions as a generator.

Regardless of the mode, an electric machine typically defines anelectromagnetic circuit having a moving portion and a stationaryportion. The moving portion or rotor having rotor windings that aredriven to rotate with respect to the stationary portion by a source ofrotation, such as a mechanical or electrical machine, which for someaircraft may be a gas turbine. The stationary portion or stator cansurround the rotor, and can comprise field magnets, such wire windingsaround a ferromagnetic iron core. The stator creates a magnetic fieldwhich passes through the rotor, exerting force on the rotor windings. Insuch electric machines, the rotor can be formed of a ferromagneticmaterial to channel magnetic flux. The rotor is conventionally arrangedusing insulated plates or laminas (typically of iron or iron alloy)assembled or stacked together to collectively form a rotor core.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is an isometric view of a gas turbine engine having a generator,in accordance with various aspects described herein.

FIG. 2 is an isometric view of an exterior of the generator of FIG. 1 ,in accordance with various aspects described herein.

FIG. 3 is a schematic cross-sectional view of the generator of FIG. 2 ,taken along line of FIG. 2 , in accordance with various aspectsdescribed herein.

FIG. 4 illustrates a partially exploded isometric view of a rotorassembly and coil support assembly for the generator of FIG. 3 , inaccordance with various aspects described herein.

FIG. 5 illustrates a simplified cross-sectional end view of the rotorassembly of FIG. 4 , with parts omitted for clarity, in accordance withvarious aspects described herein.

FIG. 6 illustrates simplified cross-sectional end view of an electricmachine in accordance with non-limiting aspects described herein.

FIG. 7 illustrates an exemplary method flowchart diagram of fabricatingan electric machine, in accordance with various aspects describedherein.

FIG. 8 illustrates a perspective view of the first coolant distributionring of the rotor assembly of FIG. 4 , in accordance with variousaspects described herein.

FIG. 8A illustrates a cross sectional view, taken along line VA-VA ofthe first coolant distribution ring of FIG. 8 , in accordance withvarious aspects described herein.

FIG. 9 illustrates a perspective view of the second coolant distributionring of the rotor assembly of FIG. 4 , in accordance with variousaspects described herein.

FIG. 10 illustrates a perspective view of the coil support disc of therotor assembly of FIG. 4 , in accordance with various aspects describedherein.

FIG. 11 illustrates a zoomed cross-sectional view of the coil supportassembly of the rotor assembly of FIG. 4 , in accordance with variousaspects described herein.

DETAILED DESCRIPTION

Aspects of the disclosure can be implemented in any environment using anelectric motor regardless of whether the electric motor provides adriving force or generates electricity. For purposes of thisdescription, such an electric motor will be generally referred to as anelectric machine, electric machine assembly, or similar language, whichis meant to clarify that one or more stator/rotor combinations can beincluded in the machine. While this description is primarily directedtoward an electric machine providing power generation, it is alsoapplicable to an electric machine providing a driving force or anelectric machine providing both a driving force and power generation.Further, while this description is primarily directed toward an aircraftenvironment, aspects of the disclosure are applicable in any environmentusing an electric machine. Thus, a brief summary of a contemplatedenvironment should aid in a more complete understanding.

Conventional electric machines typically define an electromagneticcircuit having a moving portion and a stationary portion. The movingportion, or rotor, can include rotor windings that are driven to rotatewith respect to the stationary portion by a source of rotation, such asa mechanical or electrical machine, which for some aircraft may be a gasturbine. The rotor is conventionally arranged using insulated plates orlaminas (typically of iron or iron alloy) assembled or stacked togetherto collectively form a rotor core. In such electric machines, the rotorcan be formed of a ferromagnetic material to channel magnetic flux. Thestationary portion, or stator, can surround the rotor, and can comprisefield magnets, such wire windings around a ferromagnetic iron core. Thestator creates a magnetic field which passes through the rotor, exertingforce on the rotor windings. Since the rotor core is typically formed ofiron or other conductive material, it presents low resistance or lowreluctance (designated “R”), to the flow of flux, and therefore therotor core itself contains very little magnetic energy.

Typically, an air gap must be defined between the stator and rotor toprovide a clearance to enable the rotor to turn or rotate with respectto the stator. Furthermore, the air gap between the stator and rotor canconfigured to add reluctance to the magnetic circuit which can increasethe energy storage capacity of the rotor core.

The size of the air gap between the rotor and stator can have asignificant effect on the motor's electrical characteristics. Thus, foroptimum machine performance, the size or width of the air gap must becarefully considered. Typically, it is desirable to have a relativelysmall air gap. For example, a relatively small air gap can raise theoverall circuit reluctance R by more than ten times. That is, arelatively small air gap can contain over ten times the magnetic energyof the iron core, which provides the flux conduit for channeling fluxthrough the coil and concentrates the flux at the air gap. However, assize of the air gap increases, (i.e., as the reluctance R increases) therequired magnetizing current increases, thereby adding undesired heatingeffects to the rotor windings.

Conversely, a relatively small air gap between the rotor and stator cancontribute to undesired noise and electrical losses. For example, as thesize of the air gap in a conventional generator decreases, the totalharmonic distortion (THD) of the voltage output waveform of thegenerator can increase.

While “a set of” various elements will be described, it will beunderstood that “a set” can include any number of the respectiveelements, including only one element. As used herein, the terms “axial”or “axially” refer to a dimension along a longitudinal axis of agenerator or along a longitudinal axis of a component disposed withinthe generator.

As used herein, the terms “radial” or “radially” refer to a dimensionextending between a center longitudinal axis, an outer circumference, ora circular or annular component disposed thereof. The use of the terms“proximal” or “proximally,” either by themselves or in conjunction withthe terms “radial” or “radially,” refers to moving in a direction towardthe center longitudinal axis, or a component being relatively closer tothe center longitudinal axis as compared to another component.

All directional references (e.g., radial, axial, upper, lower, upward,downward, left, right, lateral, front, back, top, bottom, above, below,vertical, horizontal, clockwise, counterclockwise) are only used foridentification purposes to aid the reader's understanding of thedisclosure, and do not create limitations, particularly as to theposition, orientation, or use thereof. Connection references (e.g.,attached, coupled, connected, and joined) are to be construed broadlyand can include intermediate members between a collection of elementsand relative movement between elements unless otherwise indicated. Assuch, connection references do not necessarily infer that two elementsare directly connected and in fixed relation to each other.

As used herein, a “wet” cavity generator includes a cavity housing therotor and stator that is exposed to free liquid coolant (e.g. coolantfreely moving within the cavity). In contrast, a “dry” cavity generatorthe rotor and stator can be cooled by coolant contained within limitedin fluidly sealed passages (e.g. non-freely moving about the cavity).

The exemplary drawings are for purposes of illustration only and thedimensions, positions, order and relative sizes reflected in thedrawings attached hereto can vary.

FIG. 1 illustrates a gas turbine engine 10 having an accessory gear box(AGB) 12 and a generator or electric machine 14 according to an aspectof the disclosure. The gas turbine engine 10 can be a turbofan engine,such as a General Electric GEnx or CF6 series engine, commonly used inmodern commercial and military aviation or it could be a variety ofother known gas turbine engines such as a turboprop or turboshaft. TheAGB 12 can be coupled to a turbine shaft (not shown) of the gas turbineengine 10 by way of a mechanical power take off 16. The gas turbineengine 10 can be any suitable gas turbine engine used in modern aviationor it could be a variety of other known gas turbine engines such as aturboprop or turboshaft. The type and specifics of the gas turbineengine 10 are not germane to the disclosure and will not be describedfurther herein. While a generator 14 is shown and described, aspects ofthe disclosure are not so limited, and aspects can include anyelectrical machine, such as, without limitation, a motor, or generator.

FIG. 2 more clearly illustrates a non-limiting example of the generator14 and its housing 18 in accordance with aspects of the disclosure. Thegenerator 14 can include a clamping interface 20, used to clamp thegenerator 14 to the AGB (not shown). Multiple electrical connections canbe provided on the exterior of the generator 14 to provide for thetransfer of electrical power to and from the generator 14. Theelectrical connections can be further connected by cables to anelectrical power distribution node of an aircraft having the gas turbineengine 10 to power various items on the aircraft, such as lights andseat-back monitors. The generator 14 can include a liquid coolant systemfor cooling or dissipating heat generated by components of the generator14 or by components proximate to the generator 14, one non-limitingexample of which can be the gas turbine engine 10. For example, thegenerator 14 can include a liquid cooling system using oil as a coolant.

The liquid cooling system can include a cooling fluid inlet port 82 anda cooling fluid outlet port 84 for controlling the supply of coolant tothe generator 14. In one non-limiting example, the cooling fluid inletand output ports 82, 84 can be utilized for cooling at least a portionof a rotor or stator of the generator 14. The liquid cooling system canalso include a second coolant outlet port 91, shown at a rotatable shaftportion of the generator 14. Optionally, by way of non-limiting example,the liquid cooling system can include a rotatable shaft coolant inletport 94 or a generator coolant outlet port 95. While not shown, aspectsof the disclosure can further include other liquid cooling systemcomponents, such as a liquid coolant reservoir fluidly coupled with thecooling fluid inlet port 82, the rotatable shaft coolant inlet port 94,the cooling fluid outlet port 84, or the generator coolant outlet port95, and a liquid coolant pump to forcibly supply the coolant through theports 82, 84, 94, 95 or generator 14.

A non-limiting interior of the generator 14 is best seen in FIG. 3 ,which is a cross-sectional view of the generator 14 shown in FIG. 2taken along line A rotatable shaft 40 is located within the generator 14and is the primary structure for supporting a variety of components. Therotatable shaft 40 can have a single diameter or one that can vary alongits length. The rotatable shaft 40 is supported by spaced bearings 42and 44 and configured to rotate about a rotational axis 41. Several ofthe elements of the generator 14 have a fixed component and a rotatingcomponent, with the fixed component fixed relative to the housing 18 andwith the rotating component being provided on, or rotatably fixedrelative to the rotatable shaft 40. Examples of these elements caninclude a main machine 50, housed within a main machine cavity 51, anexciter 60, and a permanent magnet generator (PMG) 70. The correspondingrotating component comprises a main machine rotor 52, an exciter rotor62, and a PMG rotor 72, respectively, and the corresponding fixedcomponent comprises a main machine stator 54 or stator core, an exciterstator 64, and a PMG stator 74. In this manner, the main machine rotor52, exciter rotor 62, and PMG rotor 72 are disposed on and co-rotatewith the rotatable shaft 40. The fixed components can be mounted to anysuitable part of the housing 18, and include the main machine stator 54,exciter stator 64, and PMG stator 74. Collectively, the fixed componentsdefine an interior through which the rotatable shaft 40 extends androtates relative thereto.

It will be understood that the main machine rotor 52, exciter rotor 62,and PMG rotor 72 can have a set of rotor poles, and that the mainmachine stator 54, exciter stator 64, and PMG stator 74 can have a setof stator poles. The set of rotor poles can generate a set of magneticfields relative to the set of stator poles, such that the rotation ofthe rotor magnetic fields relative to the stator poles generate currentin the respective stator components.

At least one of the rotor poles and stator poles can be formed by a corewith a post and wire wound about the post to form a winding, with thewinding having at least one end turn. Aspects of the disclosure showninclude at least one set of stator windings 90 arranged longitudinallyalong the housing 18, that is, in parallel with housing 18 and therotational axis 41. The set of stator windings 90 can also include a setof stator winding end turns 92 extending axially beyond opposing ends ofa longitudinal length of a main machine stator 54.

The components of the generator 14 can be any combination of knowngenerators. For example, the main machine 50 can be either a synchronousor asynchronous generator. In addition to the accessories shown in thisaspect, there can be other components that need to be operated forparticular applications. For example, in addition to theelectromechanical accessories shown, there can be other accessoriesdriven from the same rotatable shaft 40 such as the liquid coolant pump,a fluid compressor, or a hydraulic pump.

As explained above, the generator 14 can be oil cooled and thus caninclude a cooling system 80. The cooling oil can be used to dissipateheat generated by the electrical and mechanical functions of thegenerator 14. The cooling system 80 using oil can also provide forlubrication of the generator 14. In the illustrated aspects, thegenerator 14 can be a liquid cooled, wet cavity cooling system 80including the cooling fluid inlet port 82 and the cooling fluid outletport 84 for controlling the supply of the cooling fluid to the coolingsystem 80. The cooling system 80 can further include, for example, acooling fluid reservoir 86 and various cooling passages. The rotatableshaft 40 can provide one or more channels or paths for coolant or fluidcoolant flow 85 (shown schematically as arrows) for the main machinerotor 52, exciter rotor 62, and PMG rotor 72, as well as a rotor shaftcooling fluid outlet 88, such as the second coolant outlet port 91,wherein residual, unused, or unspent oil can be discharged from therotatable shaft 40.

In non-limiting examples of the generator 14, the fluid coolant flow 85can further be distributed, directed, exposed, sprayed, or otherwisedeposited onto the set of stator windings 90, the set of stator windingend turns 92, or onto alternative or additional components. In thisexample, the fluid coolant flow 85 can flow from the rotatable shaft 40radially outward toward the set of stator windings 90 or the set ofstator winding end turns 92. In this sense, the coolant can cool therespective set of stator windings 90 or set of stator winding end turns92.

FIG. 4 illustrates an isometric partially exploded view of a mainelectric machine rotor assembly 96. As shown, the rotor assembly 96 caninclude a rotor core 100, such as a laminated rotor core, rotatablyconnected to co-rotate with the rotatable shaft 40. The rotor core 100can be formed of ferromagnetic material (for example, iron or ironalloy) to channel magnetic flux. The rotor core 100 can comprise anannular or cylindrical body 121 defining a first bore 123 therethroughsized to receive the rotatable shaft 40. In non-limiting aspects, thefirst bore 123 can comprise a longitudinal axis corresponding to therotational axis 41 of the rotatable shaft 40.

The rotor assembly 96 can optionally include a set of coil supportassemblies 140. Each coil support assembly 140 can include a respectivefirst coolant distribution ring 141, a second coolant distribution ring142, and a retaining ring 143. In some non-limiting aspects, each coilsupport assembly 140 can further include a coil support disc 144.

The rotor assembly 96 can further define a first end 102 and a secondend 104, axially spaced from the first end 102. The rotor assembly 96can include at least one rotor pole 106. In the illustration of FIG. 4 ,an aspect comprising four rotor poles 106 is shown. Other aspects arenot so limited, and rotor assembly 96 can alternatively have fewer thanfour rotor poles 106, or more than four poles 106, without departingfrom the scope of the disclosure, and aspects can be adapted to rotorassemblies 96 having any desired number of rotor poles 106. Each rotorpole 106 can be defined by a set of conductive rotor wiring or rotorwindings 110 wound about a portion of the rotor core 100. For example,in non-limiting aspects, the rotor core 100 can define a set of slots108. In non-limiting aspects, the slots 108 can comprise a respectivelongitudinal axis extending axially along the rotor core 100.

The slots 108 can be circumferentially spaced from each other. Innon-limiting aspects, the slots 108 can be disposed about a periphery ofthe rotor core 100. The slots 108 can be adapted to receive a respectiverotor winding 110 therein. For example, each slot 108 can define acircumferential width and a radial depth sized to receive a respectiverotor winding 110 therein.

The rotor windings 110 disposed within the slots 108 can define an axialwinding portion 111 extending axially along the rotor core 100, androtor winding end turns 112 extending axially beyond the rotor core 100.In the perspective of the illustrated example, the slots 108 canunderlie the set of rotor windings 110. While the rotor windings 110 orthe rotor winding end turns 112 can refer to a set of or plural windingsor end turns, an end turn can include only one of the set of rotorwindings 110, or only one portion of the set of rotor windings 110extending axially beyond the rotor core 100, such as only at the firstend 102 or the second end 104.

The set of rotor winding end turns 112 can define respective loops orarcuate bight portions 113 disposed axially beyond the rotor core 100.In non-limiting aspects, each bight portion 113 can define a respectivechannel 116 extending therethrough. For example, in non-limiting aspectseach respective channel 116 can have a width defined by a width andspacing between the slots 108.

In non-limiting aspects, the coil support assembly 140 can be disposedat either end 102, 104 of the rotor assembly 96. For example, in someaspects, a single coil support assembly 140 can be disposed at one endof the rotor assembly 96. In other non-limiting aspects, a respectivecoil support assembly 140 can be disposed at each end of the rotorassembly 96.

A respective coil support assembly 140 can be fixedly coupled to eachend of the rotatable shaft 40 of the rotor assembly 96. For example, arespective coil support assembly 140 can be coupled to one end (e.g.,the first end 102 or the second end 104) of the rotor assembly 96. Inother aspects, a respective coil support assembly 140 can be coupled tothe rotatable shaft 40 at both the first end 102 and the second end 104of the rotor assembly 96.

FIG. 5 depicts a simplified end view of the rotor core 100 with therotor windings 110 and other parts omitted for ease of description andunderstanding. As shown, the rotor core 100 can include a set of rotorteeth 125 extending outwardly therefrom, for example along acircumferential peripheral surface 230 of the rotor core 100. Each rotortooth 125 can comprise a respective first distal tip 125 a defined atthe radial extent of the rotor tooth 125. Each rotor tooth 125 candefine a respective first radial length L1, extending radially from acenter point (designated “CP”) of the rotor core 100 to the first distaltip 125 a of the respective rotor tooth 125. In non-limiting aspects,each tooth can have the same length such that the respective firstradial length L1 of each rotor tooth 125 can be identical. Innon-limiting aspects, the set of rotor teeth 125 can becircumferentially spaced from each other about the rotor core 100 todefine the slots 108 therebetween. For example, each slot 108 can bedefined between two respective immediately adjacent rotor teeth 125.

In non-limiting aspects, the rotor core 100 can include a set of slottedportions 227 and a set of non-slotted portions 228. Each slotted portion227 can be defined by a respective subset of the set of rotor teeth 125and comprise the slots 108 sized to receive a respective rotor winding110 therein. In non-limiting aspects, the number of slotted portions 227can correspond to the number of rotor poles 106 of the rotor core 100.It will be appreciated that each slotted portion 227 can comprise arespective radial length corresponds to, or is equal to, a respectivefirst radial length L1 of the rotor teeth 125 forming the respectiveslotted portion 227. In non-limiting aspects, the non-slotted portion228 can define a respective second radial length L2, extending radiallyfrom the center point CP of the rotor core 100 to a peripheral tip 231of the non-slotted portion 228 at the circumferential peripheral surface230 of the rotor core 100. As illustrated, the center point CP cancorrespond to the rotational axis 41 of the rotatable shaft 40. Innon-limiting aspects, the first radial length L1 defined by the slottedportion 227 can be less than the radial length defined by thenon-slotted portion 228. In this way, non-limiting aspects of the rotorcore 100 can define a first diameter D1 with respect to at least oneslotted portion 227, and a second circumference diameter D2 with respectto at least on non-slotted portion 228. In some aspects, the firstdiameter D1 can be less than the second diameter D2.

In the non-limiting aspect illustrated, the rotor core 100 is afour-pole type rotor core, with each pole 106 comprising a respective aslotted portion 227 of the rotor core 100. It will be appreciated thataspects as disclosed herein are not limited to any specific number ofrotor poles, and aspects can be adapted to rotor assemblies 96 havingany desired number of rotor poles 106.

A non-limiting aspect of a portion of the electric machine 14 isdepicted in simplified cross-sectional end view in FIG. 6 with partsomitted (e.g., rotatable shaft 40, the rotor windings 110, etc) for easeof description and understanding.

In aspects, a stator core 354 can comprise a cylindrical inner periphery355 defining a second bore 356. The rotor core 100 is sized to berotatably disposed or received within the second bore 356. A set ofinwardly facing stator teeth 346 can be can be evenly spaced about thecylindrical inner periphery 355 to define a set of stator slots 347therebetween. Each stator tooth 346 can comprise a respective seconddistal tip 346 a defined at the radially inwardly facing extent of thetooth 346. As illustrated, each first distal tip 125 a of the set ofrotor teeth 125 can rotatably oppose a respective second distal tip 346a of the set of stator teeth 346.

In non-limiting aspects, as shown, air gap 360 can be defined betweenthe rotor core 100 and the stator core 354. The air gap 360 can define afirst radial width 361 defined between a respective slotted portion 227of the rotor core 100, and the cylindrical inner periphery 355 of thestator core 354. For example, the first radial width 361 can be measuredbetween the first distal tip 125 a of a respective rotor tooth 125 at aslotted portion 227 of the rotor core 100, and an opposing second distaltip 346 a of an opposing stator tooth 346 at the cylindrical innerperiphery 355 of the stator core 354. In non-limiting aspects, the firstradial width 361 can extend along the longitudinal length of the rotorcore 100. In non-limiting aspects, the air gap 360 can further define asecond radial width 362 defined between a respective non-slotted portion228 of the rotor core 100 and the cylindrical inner periphery 355 of thestator core 354. For example, the second radial width 362 can bemeasured between the first distal tip 125 a of a respective rotor tooth125 at a slotted portion 227 of the rotor core 100, and an opposingsecond distal tip 346 a of an opposing stator tooth 346 at thecylindrical inner periphery 355 of the stator core 354.

In non-limiting aspects, the second radial width 362 can extend alongthe longitudinal length of the rotor core 100. In aspects the firstradial width 361, can be wider than the second radial width 362. Forexample, in non-limiting aspects, the respective first radial length L1of each rotor tooth 125 in a respective slotted portion 227 can be lessthan the second radial length L2 of the non-slotted portion 228. In suchaspects, the arrangement of the first radial width 361 being larger orwider than the second radial width 362 has the particular purpose ofinfluencing the reluctance of a flux path between the rotor core 100 andstator core. In operation, the first radial width 361 of the air gap 360being wider than the second radial width 362 enables an improveddistribution of the magnetic flux along the flux path between the rotorand stator resulting in a reduction in the total harmonic distortion ofthe output voltage waveform of the generator 14.

FIG. 7 illustrates a method 400 of fabricating an electric machine 14 inaccordance with the disclosure herein. Non-limiting aspects of themethod 400 can include, at 402, arranging a rotor core 100 comprising acylindrical body 121 having an outwardly facing peripheral surface 230.The arranging a rotor core 100 can include at 404, defining a slottedportion 227 of the rotor core 100 having slots 108 defined by a set ofrotor teeth 125 projecting outwardly from the peripheral surface 230,such that each rotor tooth 125 comprises a respective first distal tip125 and a respective first radial length L1 extending radially from acenter point CP of the rotor core 100 to the first distal tip 125 a.

The arranging a rotor core can further include, at 405 defining anon-slotted portion 228 on the peripheral surface 230 to define arespective second radial length L2. The second radial length can extendradially from the center point CP of the rotor core 100 to theperipheral surface 230. In non-limiting aspect, the first radial lengthL1 is less than the second radial length L2.

The method 400 can further comprise, at 410, arranging a stator core354, having cylindrical inner periphery 355 defining a second bore 356.The arranging a stator core 354 can include defining a set of inwardlyfacing stator teeth 346 spaced about the cylindrical inner periphery 355to define a set of stator slots 347 therebetween. In non-limitingaspects, each stator tooth 346 can comprise a respective second distaltip 346 a defined at a radially inwardly facing extent of the statortooth 346. The method 400 can include at 415 disposing the rotorassembly 96 within the second bore 356.

Non-limiting aspects of the method 400 can further comprise at 420defining an air gap 360 between the rotor core 100 and the stator core354. In non-limiting aspects, the air gap 360 can comprises a firstradial width 361 defined between a respective slotted portion 327 andthe cylindrical inner periphery 355 of the stator core 354, and a secondradial width 362 defined between a respective non-slotted portion 228and the cylindrical inner periphery 355 of the stator core 354.

For example, in some aspects, the first radial width 361 can be definedbetween the first distal tip 125 a of a respective rotor tooth 125 at aslotted portion 127 of the rotor core 100, and a second distal tip 346 aof an opposing stator tooth 346 at the cylindrical inner periphery 355of the stator core 354. In such aspects, the second radial width 362 canbe defined between the peripheral surface 230 of the non-slotted portion238 of the rotor core 100 and a second distal tip 346 a of an opposingstator tooth 346 at the cylindrical inner periphery 355 of the statorcore 354. In non-limiting aspects, the first radial width 361 can begreater than the second radial width

The sequence depicted is for illustrative purposes only and is not meantto limit the method 400 in any way as it is understood that the portionsof the method can proceed in a different logical order, additional orintervening portions can be included, or described portions of themethod can be divided into multiple portions, or described portions ofthe method can be omitted without detracting from the described method.

A non-limiting aspect of the first coolant distribution ring 141 isdepicted in FIG. 8 and FIG. 8A. In non-limiting aspects, the firstcoolant distribution ring 141 can comprise a generally annular memberhaving an inwardly facing or first radially inner surface 146 and anopposing, outwardly facing or first radially outer surface 147. Thefirst radially inner surface 146 can define a bore 148 sized to receivethe rotatable shaft 40 therethrough and to receive a flow of coolanttherefrom. As such, the first radially inner surface 146 can operativelydefine a coolant collection surface. In some aspects, the first radiallyinner surface 146 can be a relatively smooth surface. In othernon-limiting aspects, the first radially inner surface 146 can define aset of channels or first grooves 149 thereon. The first grooves 149 canbe arranged in fluid communication with the rotatable shaft 40. As suchthe set of first grooves 149 can cooperatively define a coolantreservoir.

The first coolant distribution ring 141 can further include a set offirst channels 145 defined therethrough. In non-limiting aspects, thefirst channels 145 can be circumferentially spaced about the firstcoolant distribution ring 141. The first channels 145 can compriserespective longitudinal axes that extend radially through the firstcoolant distribution ring 141. The first channels 145 can be sized toallow a flow of cooling fluid therethrough. For example, each firstchannel 145 can extend radially from a first end 145 a disposed at thefirst radially inner surface 146 to an opposing second end 145 b at thefirst radially outer surface 147. Each first channel 145 can comprise afirst coolant inlet 184 defined on the first radially inner surface 146,and a corresponding first coolant outlet 186 defined on the firstradially outer surface 147, at the opposing second end 145 b.

In operation, the first channels 145 can be in fluid communication withthe rotatable shaft 40, or the set of first grooves 149, or both, toreceive the flow of coolant therefrom. In non-limiting aspects, thefirst channels 145 can further be in fluid communication with arespective first coolant outlet 186. At least a subset of the coolantoutlets 186 can define a respective spray nozzle 190 at a radiallydistal end. The spray nozzles 190 can be disposed at circumferentiallyspaced intervals on the first radially outer surface 147.

The first coolant distribution ring 141 can be fixedly coupled to therotatable shaft 40 using one or more bolts, screws, pins, keys, or otherknown fasteners. In other non-limiting aspects, the first coolantdistribution ring 141 can be coupled to the rotatable shaft 40 via aninterference, friction, or press-fit engagement between the firstcoolant distribution ring 141 and the rotatable shaft 40. For example,the first radially inner surface 146 can be fixedly coupled to therotatable shaft 40. Other aspects are not so limited, and it iscontemplated that first coolant distribution ring 141 can be rotatablycoupled to the rotatable shaft 40 by any desired affixing mechanisms. Itwill be appreciated that when so coupled, a rotation of the rotatableshaft 40 will result in rotation of the first coolant distribution ring141.

In non-limiting aspects, the first coolant distribution ring 141 canfurther comprise a set of first tabs 153 extending radially therefrom.In some aspects, the first tabs 153 can be circumferentially spacedabout the first coolant distribution ring 141. In some non-limitingaspects, the number of first tabs 153 can be equal to the number ofpoles of the generator 14. Other aspects are not so limited, and thefirst coolant distribution ring 141 can comprise any desired number offirst tabs 153. For example, it will be appreciated that aspects asdisclosed herein are not limited to any specific number of rotor poles,and aspects can be adapted to rotor assemblies 96 having any desirednumber of poles.

With reference back to FIG. 4 , in non-limiting aspects, the firstcoolant distribution ring 141 can be disposed to at least partiallyunderlie the rotor winding end turns 112. In this example, “underlie”denotes a relative position radially closer to the rotational axis 41 ofthe rotatable shaft 40. In non-limiting aspects, the first radiallyouter surface 147 can be arranged proximal to and facing the coilwinding end turns 112.

With continued reference to FIG. 4 , in non-limiting aspects, theretaining ring 143 can at least partially overlie the rotor winding endturns 112. In this example, “overlie” denotes a relative positionradially farther from the rotational axis 41 of the rotatable shaft 40.

In non-limiting aspects, the retaining ring 143 can be disposed tosurround or enclose the first coolant distribution ring 141. Forexample, in non-limiting aspects, the retaining ring 143 can overlie orradially surround the first coolant distribution ring 141. Innon-limiting aspects, the retaining ring 143 can optionally comprise aset of second tabs 167 extending radially therefrom. In some aspects,the second tabs 167 can be circumferentially spaced about the retainingring 143. In non-limiting aspects, the number of second tabs 167 can beequal to the number of poles of the generator 14. Other aspects are notso limited, and the retaining ring 143 can comprise any desired numberof second tabs 167.

The retaining ring 143 can be rigidly or fixedly coupled to the firstcoolant distribution ring 141. For example, the retaining ring 143 canbe coupled to the first coolant distribution ring 141 using bolts orother fasteners disposed through the set of second tabs 167. In othernon-limiting aspects, the retaining ring 143 can be coupled to the firstcoolant distribution ring 141 using bolts or other fasteners disposedthrough the set of first tabs 153. In still other non-limiting aspectsthe retaining ring 143 can be coupled to the first coolant distributionring 141 using bolts or other fasteners disposed through both the set offirst tabs 153 and the set of second tabs 167. When so coupled, theretaining ring 143 can limit or arrest an axial movement of the firstcoolant distribution ring 141. In aspects the retaining ring 143 andfirst coolant distribution ring 141 can cooperatively limit a radialdeflection or relative movement of the rotor winding end turns 112.

In non-limiting aspects, the retaining ring 143 can be further arrangedto surround the second coolant distribution ring 142. For example, theretaining ring 143 can be arranged be to at least partially overlie thesecond coolant distribution ring 142. In this example, “overlie” denotesa relative position radially farther from the rotational axis 41 of therotatable shaft 40. In non-limiting aspects, the retaining ring 143 cancomprise a radially inner surface 151 defining a bore 175.

A non-limiting aspect of the second coolant distribution ring 142 isdepicted in FIG. 9 . The second coolant distribution ring 142 caninclude an inwardly facing second radially inner surface 152 defining abore 155. The second coolant distribution ring 142 can further includean outwardly facing, second radially outer surface 157 opposing thesecond radially inner surface 152. The radially inner surface 151 of theretaining ring 143 can be disposed to face the radially outer surface157 of the second coolant distribution ring 142. The second radiallyinner surface 152 can define a set of second grooves 158 and a set ofthird grooves 159. In nonlimiting aspects, the second grooves 158 cancomprise an axially extending longitudinal axis. In aspects, the secondgrooves 158 can be circumferentially spaced from each other. The thirdgrooves 159 can comprise a circumferentially extending longitudinalaxis. The third grooves 159 can be axially spaced from each other. Innon-limiting aspects, the second grooves 158 and third grooves 159 canbe arranged at an angle with respect to each other. For example, asillustrated, in some aspects the second grooves 158 can be arrangedorthogonally with respect to the third grooves 159. In other aspects thesecond grooves 158 can be arranged at any desired angle (i.e.,non-parallel) with respect to the third grooves 159. In operation, thesecond grooves 158 and third grooves 159 of the second coolantdistribution ring 142 can be arranged in fluid communication with theset of first grooves 149 of the first coolant distribution ring 141 toreceive the fluid coolant flow 85 therefrom.

The second coolant distribution ring 142 can further define a set ofsecond channels 172 extending therethrough. In non-limiting aspects, thesecond channels 172 can extend radially between the second radiallyinner surface 152 and the second radially outer surface 157. In someaspects, the second channels 172 can be circumferentially spaced fromeach other about the second coolant distribution ring 142. The secondchannels 172 can be in fluid communication with at least one of the setof second grooves 158 and the set of third grooves 159 to receive a flowof coolant therefrom. The second channels 172 can comprise a respectivelongitudinal axis that extends radially through the second coolantdistribution ring 142. The second channels 172 can be sized to allow thefluid coolant flow 85 to flow therethrough. For example, each radiallyextending second channel 172 can extend radially from a first end 172 adisposed at the second radially inner surface 152 to an opposing secondend 172 b disposed at the second radially outer surface 157. Each secondchannel 172 can comprise a second coolant inlet 173 defined on the firstradially inner surface 146 in fluid communication with at least one ofthe set of second grooves 158 and the set of third grooves 159. Eachsecond channel 172 can further comprise a corresponding second coolantoutlet 174 defined on the second radially outer surface 157, at theopposing second end 172 b.

With reference to FIG. 4 , in non-limiting aspects, the second coolantdistribution ring 142 can be disposed to at least partially overlie therotor end turns 112 relative to a rotational axis of the rotor. Thesecond coolant distribution ring 142 can be in fluid communication withthe rotor winding end turns 112. In non-limiting aspects, the secondcoolant distribution ring 142 can be disposed radially between the endturn windings 112 and the retaining ring 143. In non-limiting aspects,the second coolant distribution ring 142 can be disposed to underlie theretaining ring 143.

FIG. 10 depicts a non-limiting aspect of the coil support disc 144. Innon-limiting aspects, the coil support disc 144 can comprise an annularmember or disc having a first face 181 and an opposing second face 182.For example, the first face 181 can be axially inwardly facing, e.g.disposed to face the rotor core 100. In aspects, the second face 182 canbe axially outwardly facing, e.g., disposed to face away from the rotorcore 100. The coil support disc 144 can comprise a radially inwardlyfacing, or third radially inner surface 176 (i.e., facing the rotatableshaft 40) and an opposing radially outwardly facing, or third radiallyouter surface 177 (i.e., facing outward from the rotatable shaft 40).The third radially inner surface 176 and third radially outer surface177 can be arranged between the first face 181 and second face 182.

In non-limiting aspects, the third radially outer surface 177 can definea set of notches 185 or gaps defined therethrough. The notches 185 canbe circumferentially spaced from each other about the coil support disc144. The notches 185 can be sized to operatively receive a respectiverotor winding 110 axially therein. In this way, the coil support disc144 can support, or otherwise limit a lateral movement of each rotorwinding 110 disposed within a respective notch 185.

The third radially inner surface 176 can define a bore 178 sized toreceive the rotatable shaft 40 therethrough. In non-limiting aspects,the third radially inner surface 176 can be arranged in fluidcommunication with the rotatable shaft 40 to receive a fluid coolantflow 85 therefrom. As such, the third radially inner surface 176 canoperatively provide a coolant collection surface. In some aspects, thethird radially inner surface 176 can be a relatively smooth surface. Inother non-limiting aspects, the third radially inner surface 176 candefine a set of fourth grooves 179 or notches thereon. The thirdradially inner surface 176 or the fourth grooves 179 or both, can be influid communication with the rotatable shaft 40. As such, third radiallyinner surface 176 or the fourth grooves 179 or both, can operativelyprovide a coolant reservoir.

The coil support disc 144 can further include a set of third channels183 defined therethrough. In non-limiting aspects, at least a portion ofthe third channels 183 can be circumferentially spaced from each otherabout the coil support disc 144. Each third channel 183 can define arespective path extending radially through the coil support disc 144. Insome non-limiting aspects, a fourth channel 187 can be defined in fluidcommunication with at least a subset of the third channels 183. In thissense, at least a subset of the third channels 183 can be in fluidcommunication with each other via the fourth channel 187. For example,the fourth channel 187 can be arranged to extend circumferentially aboutthe coil support disc 144 and coupled to at least a subset of the thirdchannels 183. The third channels 183 and fourth channel 187 can be sizedto allow a flow of cooling fluid therethrough. At least a subset of thethird channels 183 can extend or traverse from a first end 183 a at thethird radially inner surface 176 to an opposing second end 183 b. Innon-limiting aspects the second end 183 b can be disposed at the thirdradially outer surface 177. For example, the second end 183 b can bedisposed within a respective notch 185. In this sense, the thirdradially inner surface 176 can be in fluid communication with the set ofnotches 185 via the set of third channels 183. It will be appreciatedthat because the third radially inner surface 176 and set of thirdchannels 183 can be in fluid communication with the set of notches 185,the third radially inner surface 176 and set of third channels 183 canfurther be in fluid communication with the respective winding 110disposed in the respective notch 185. In other non-limiting aspects, thesecond end 183 b can be disposed on the first face 181. Each thirdchannel 183 can comprise a third coolant inlet 188. In non-limitingaspects, the third coolant inlet 188 can be defined on the firstradially inner surface 146. In some aspects, each third channel 183 canfurther comprise a third coolant outlet 189 at the opposing second end183 b. In non-limiting aspects, the third coolant outlet 189 can bedefined on the first face 181. In other aspects, the third coolantoutlet 189 can be defined on the third radially outer surface 177. Forexample, in non-limiting aspects the third coolant outlet 189 can bedefined within a respective notch 185. In some aspects, the thirdcoolant outlets 189 can be disposed at circumferentially spacedintervals on the first face 181. In aspects, the second channels 172 canbe in fluid communication with the rotatable shaft 40, or the set offirst grooves 149, or both, to receive the fluid coolant flow 85therefrom. In this way, the fluid coolant flow 85 can be operativelycentrifugally conveyed from the third radially inner surface 176 to theset of third channels 183 and to the set of notches 185. In non-limitingaspects, the fluid coolant flow 85 through the set of third channels canbe in parallel with the fluid coolant flow 85 through the first coolantdistribution ring 141, or the second coolant distribution ring 142, orboth.

The coil support disc 144 can be fixed to the rotatable shaft 40 usingone or more bolts, screws, pins, keys, or other known fasteners. Inother non-limiting aspects, the coil support disc 144 can be coupled tothe rotatable shaft 40 via an interference, friction, or press-fitengagement between the coil support disc 144 and the rotatable shaft 40.Other aspects are not so limited, and it is contemplated that the coilsupport disc 144 can be rotatably coupled to the rotatable shaft 40 byany desired affixing mechanisms. It will be appreciated that when socoupled, a rotation of the rotatable shaft 40 will result in rotation ofthe first coolant distribution ring 141.

In non-limiting aspects, the coil support disc 144 can be operativelydisposed between the rotor core 100 and the first coolant distributionring 141. For example, in non-limiting aspects, the coil support disc144 can be disposed within one or more of the channels 116 defined bythe rotor winding end turns 112.

FIG. 11 illustrates a portion of the rotor assembly 96 of FIG. 4 forbetter understanding the cooling system 80 and fluid coolant flow 85from the rotatable shaft 40 to the set of rotor winding end turns 112and the set of stator winding end turns 92. As will be described in moredetail herein, the fluid coolant flow 85 can be channeled or conveyed tothe coolant winding end turns 112 via the first coolant distributionring 141, or the end coil support disc 144, or both.

The rotatable shaft 40 defines a first coolant conduit 150 fluidlyconnected with a source of coolant 165. The source of coolant 165 canbe, but is not limited to the cooling fluid inlet port (not shown). Thedirection or location of the source of coolant 165 is not limited by theillustration and can be considered in any location that is fluidlycoupled to the first coolant conduit 150. It is further considered thatadditional conduit, pumps, valves, or other devices can be included tofluidly connect the source of coolant 165 and the first coolant conduit150.

Fluid can enter the rotatable shaft 40 of the rotor assembly 96 via theinlet port 82. The rotatable shaft 40 at least in part, can define thefirst coolant conduit 150, through which fluid can flow radially outwardfrom the rotational axis 41 due to the centrifugal force effects of therotatable shaft 40. A first radial coolant passage 154, by way ofextending radially through the rotatable shaft 40, can fluidly couplethe first coolant conduit 150 and the first coolant distribution ring141.

The first coolant distribution ring 141 can receive the coolant fluidfrom the first radial coolant passage 154 via the first radially innersurface 146. For example, the coolant fluid can collect or accumulate inthe set of first grooves 149 defined on the first radially inner surface146, and then be centrifugally conveyed to the first channels 145, andto a respective first coolant outlet 189. The first coolant outlets 189can comprise a respective spray nozzle 190 at a radially distal end. Thespray nozzles 190 can be directed to provide a flow of coolant fluidradially outwardly therefrom.

In some non-limiting aspects, a first cavity 161 can be cooperativelydefined by the rotatable shaft 40, or the first coolant distributionring 141, or both, and the rotor winding end turns 112. The first cavity161 can underlie at least a portion of the rotor winding end turns 112.For example, in non-limiting aspects, each first cavity 161 can bedefined between the rotor coil end turns 112 and the first radiallyouter surface 147. In aspects, each first cavity 161 can be disposedrelative to and underlying one of the sets of rotor winding end turns112

In some non-limiting aspects, a second cavity 162 can be cooperativelydefined by the second radially outer surface 157 of the second coolantdistribution ring 142, and the retaining ring 143. In some aspects, thesecond cavity 162 can be coupled in fluid communication with the secondchannels 172 of the second coolant distribution ring 142 to operativelyreceive the fluid coolant flow 85 therefrom.

In non-limiting aspects, a gap or coolant outlet 163 can cooperativelybe defined by the coil support assembly 140 and the rotor core 100. Innon-limiting aspects, the coolant outlet 163 can be in fluidcommunication with the set of second channels 172 of the second coolantdistribution ring 142. The coolant outlet 163 can be disposed at anouter circumference 164 of the rotor assembly 96. Optionally, thecoolant outlet 163 can be a nozzle 166 configured to direct coolanttoward the set of stator windings 90 or the set of stator winding endturns 92. The coolant outlet 163 or the nozzle 166 can be at leastpartially defined by, in contact with, or coupled to an insulating layer128 located axially between at least part of the rotor core 100 and thecoil support disc 144.

As shown, the rotor winding end turns 112 can include a set of radialrotor end turn passages 156. As used herein, the set of radial rotor endturn passages 156 refers to a set of radially extending passages betweenthe rotor windings 110 that can fluidly couple the first cavity 161 tothe second coolant distribution ring 142 or the second cavity 162 orboth. For example, in non-limiting aspects, the rotor end turn passages156 can include the respective channel 116 extending through a bightportion 113 defined by a respective rotor winding end turn 112

In one non-limiting example, the first cavity 161 can be configured tooverlie the coolant fluid output volume from the first radial coolantpassage 154, the set of nozzles 190, or both, such that fluid expelledfrom the first radial coolant passage 154 or set of nozzles 190 isreceived by the first cavity 161. The first cavity 161, can beconfigured to limit or restrict where fluid received from the firstradial coolant passage 154 or set of nozzles 190 traverses radially,axially, or a combination thereof, such that the fluid is reliablydelivered radially from the first cavity 161 to the rotor winding endturns 112 or the radial rotor end turn passages 156, and then to thesecond coolant distribution ring 142 via the second cavity 162.

The set of second grooves 158 and the set of third grooves 159 of thesecond coolant distribution ring 142 can be in fluid communication withthe radial rotor end turn passages 156 to receive the fluid coolant flow85 therefrom. In operation, the coolant fluid can be centrifugallyconveyed from the second grooves 158 and third grooves 159 to the set ofsecond channels 172 and to the respective second coolant outlet 174. Thecoolant fluid can then be conveyed toward the coolant outlet 163, viathe second cavity 162, such as in an axially inward direction (e.g.,toward the rotor core 100), to the coolant outlet 163.

The coolant outlet 163 can receive the fluid coolant flow 85 from one ormore second channels 172 via the second cavity 162. For example, innon-limiting aspects, the second cavity 162 can be in fluidcommunication with the coolant outlet 163 via the radially inner surface151 such that the rotation of the rotatable shaft 40 about therotational axis 41 radially expels the fluid coolant flow 85 past therotor winding end turns 112 and radially outward from the rotor assembly96.

Additionally, in non-limiting aspects, the coil support disc 144 canreceive the coolant fluid from the first radial coolant passage 154 viathe third radially inner surface 176. For example, the coolant fluid cancollect or accumulate in the one or more fourth grooves 179. Inoperation, the coolant fluid can be centrifugally conveyed from the oneor more fourth grooves 179 to the set of third channels 183 and to therespective coolant outlet 189. In non-limiting aspects, at least asubset of the coolant outlets 189 can be in fluid communication with arespective notch 185 and the rotor winding end turn 112 disposedtherein. In some non-limiting aspects, at least a subset of the coolantoutlets 189 can be in fluid communication with a respective slot 108defined in the rotor core 100.

During operation of the generator 14, the rotation of the magnetic fieldgenerated by the set of main machine rotor windings 110 relative to theset of main machine stator windings 90 generates electricity in the mainmachine stator windings 90. This magnetic interaction further generatesheat in the set of main machine rotor windings 110 and main machinestator windings 90. In accordance with aspects described herein, coolantfluid can enter the rotatable shaft 40 of the rotor assembly 96 via theinlet port 82. The rotatable shaft 40 at least in part, can define thefirst coolant conduit 150, through which fluid can flow radially outwardfrom the rotational axis 41. Fluid from the first coolant conduit 150can pass through the first radial coolant passage 154 to be radiallyreceived by the coil support disc 144 to be radially received at thenotches 185 defined thereon and in contact with the rotor windings 110disposed in the notches 185. This contacting can remove heat from therotor windings 110 into the coolant. The coolant can then be expelledaxially into passages defined in the rotor core 100. Additionally, oralternatively, fluid from the first coolant conduit 150 can pass throughthe first radial coolant passage 154 to be radially received by thefirst coolant distribution ring 141 and distributed to the first cavity161. Fluid can continue to flow radially outward through the firstcavity 161 and through the radial rotor end turn passages 156 that passbetween the rotor windings 110 to thereby transfer heat from the set ofmain machine rotor windings 110 into the coolant by conduction. Thecoolant can be radially expelled from radial rotor end turn passages 156into the second cavity 162, where it further can collect at the radiallyinner surface 151. The radially inner surface 151 can redirect the fluidcoolant flow 85 to the coolant outlet 163, where it is further radiallyexpelled outward to contact the set of main machine stator windings 90.This contacting further removes heat from the main machine statorwindings 90 into the coolant.

The aspects disclosed herein provide method and apparatus for electricmachine operations (e.g. motor or generator operations). One advantagethat may be realized in the above aspects is that the above describedaspects have significantly improved total harmonic distortion of thevoltage output signal over conventional generators. Additionally, asdisclosed herein, improved thermal conduction to remove heat from theset of rotor windings or the set of rotor winding end turns isdisclosed. The improved thermal conductivity between the set of rotorwinding end turns and the coolant conduits coupled with the coolantchannels provide for heat removal in a much more effective fashion fromthe rotor winding end turns to the coolant.

The increased thermal dissipation of the rotor winding end turns allowsfor a higher speed rotation, which may otherwise generate too much heat.The higher speed rotation may result in improved power generation orimproved generator efficiency without increasing generator size. Thedescribed aspects having the fluid channels for the wet cavity machineare also capable of cooling the stator windings or end turn segmentswhich further reduces thermal losses of the electric machine. Reducedthermal losses in the electric machine allows for greater efficiency andgreater power density of the generator.

When designing aircraft components, reliability is also informantfeature. The above described end assembly can provide additional physicsstability and improved performance and reliability.

Many other possible aspects and configurations in addition to that shownin the above figures are contemplated by the present disclosure.Additionally, the design and placement of the various components such asvalves, pumps, or conduits can be rearranged such that a number ofdifferent in-line configurations could be realized.

To the extent not already described, the different features andstructures of the various aspects can be used in combination with eachother as desired. That one feature cannot be illustrated in all of theaspects is not meant to be construed that it cannot be, but is done forbrevity of description. Thus, the various features of the differentaspects can be mixed and matched as desired to form new aspects, whetheror not the new aspects are expressly described. Combinations orpermutations of features described herein are covered by thisdisclosure.

This written description uses examples to disclose aspects of thedisclosure, including the best mode, and also to enable any personskilled in the art to practice aspects of the disclosure, includingmaking and using any devices or systems and performing any incorporatedmethods. The patentable scope of the disclosure is defined by theclaims, and can include other examples that occur to those skilled inthe art. Such other examples are intended to be within the scope of theclaims if they have structural elements that do not differ from theliteral language of the claims, or if they include equivalent structuralelements with insubstantial differences from the literal languages ofthe claims. Further aspects of the invention are provided by the subjectmatter of the following clauses:

What is claimed is:
 1. A rotor assembly for an electric machinecomprising: a rotor core comprising a cylindrical body defining anoutwardly facing peripheral surface comprising a slotted portion havinga set of slots defined by a set of rotor teeth projecting outwardly fromthe peripheral surface, each rotor tooth comprising a respective firstdistal tip and a respective first radial length extending radially froma center point of the rotor core to the first distal tip, the peripheralsurface further comprising a non-slotted portion defining a secondradial length, extending radially from the center point of the rotorcore to the outwardly facing peripheral surface, the second radiallength being uniform throughout the non-slotted portion; wherein thefirst radial length is less than the second radial length.
 2. The rotorassembly of claim 1, wherein the rotor assembly further comprises arotatable shaft, the rotor core being coupled to the rotatable shaft. 3.The rotor assembly of claim 1, wherein the slots are sized to receive arespective rotor winding therein.
 4. The rotor assembly of claim 3,further comprising a number of slotted portions corresponding to anumber of rotor poles of the electric machine.
 5. An electric machinecomprising: a rotor assembly comprising rotor core having a cylindricalbody defining an outwardly facing peripheral surface comprising a set ofslotted portions having a set of slots defined by a set of rotor teethprojecting outwardly from the peripheral surface, each rotor toothcomprising a respective first distal tip and a respective first radiallength extending radially from a center point of the rotor core to thefirst distal tip, the peripheral surface further comprising anon-slotted portion defining a second radial length, extending radiallyfrom the center point of the rotor core to the outwardly facingperipheral surface, the second radial length being uniform throughoutthe non-slotted portion; and wherein the first radial length is lessthan the second radial length.
 6. The electric machine of claim 5,wherein the rotor assembly further comprises a rotatable shaft, therotor core being coupled to the rotatable shaft.
 7. The electric machineof claim 5, wherein the slots are sized to receive a respective rotorwinding therein.
 8. The electric machine of claim 7, wherein the rotorcore further comprises a number of slotted portions corresponding to anumber of rotor poles of the electric machine.
 9. The electric machineof claim 5, further comprising: a stator core having cylindrical innerperiphery defining a bore, and a set of stator teeth spaced about thecylindrical inner periphery defining a set of stator slots therebetween,wherein the rotor assembly is disposed within the bore.
 10. The electricmachine of claim 9, wherein each stator tooth comprises a respectivesecond distal tip defined at a radially inwardly facing peripheralsurface of the stator tooth.
 11. The electric machine of claim 10,wherein each first distal tip rotatably opposes a respective seconddistal tip.
 12. The electric machine of claim 10, further comprising anair gap defined between the rotor core and the stator core.
 13. Theelectric machine of claim 12, wherein the air gap comprises a firstradial width defined between a respective slotted portion and thecylindrical inner periphery of the stator core, and a uniform secondradial width defined between a respective non-slotted portion and thecylindrical inner periphery of the stator core.
 14. The electric machineof claim 13, wherein the first radial width is defined between the firstdistal tip of a respective tooth at a slotted portion of the rotor core,and a second distal tip of an opposing stator tooth at the cylindricalinner periphery of the stator core.
 15. The electric machine of claim14, wherein the first radial width is greater than the second radialwidth.
 16. A method of fabricating an electric machine comprising:arranging a rotor core comprising a cylindrical body having an outwardlyfacing peripheral surface, the arranging the rotor core including:defining a slotted portion of the rotor core having slots defined by aset of rotor teeth projecting outwardly from the peripheral surface,such that each rotor tooth comprises a respective first distal tip and arespective first radial length extending radially from a center point ofthe rotor core to the first distal tip; and defining a non-slottedportion on the peripheral surface to define a second radial lengthextending radially from the center point of the rotor core to theperipheral surface, the second radial length being uniform throughoutthe non-slotted portion, such that the first radial length is less thanthe second radial length.
 17. The method of claim 16, further comprisingarranging a stator core having a cylindrical inner periphery defining abore, the arranging the stator core including defining a set of statorteeth spaced about the cylindrical inner periphery to define a set ofstator slots therebetween, wherein each stator tooth comprises arespective second distal tip defined at a radially inwardly facingperipheral surface of the stator tooth; and disposing the rotor corewithin the bore.
 18. The method of claim 17, further comprising definingan air gap between the rotor core and the stator core.
 19. The method ofclaim 18, wherein the air gap comprises a first radial width definedbetween a respective slotted portion and the cylindrical inner peripheryof the stator core, and a second radial width defined between arespective non-slotted portion and the cylindrical inner periphery ofthe stator core.
 20. The method of claim 19, wherein the first radialwidth is greater than the second radial width.